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SubjectRe: [PATCH for v4.2 v18 1/3] sys_membarrier(): system-wide memory barrier (generic, x86)
On Sat, 16 May 2015 19:48:18 -0400 Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

> Here is an implementation of a new system call, sys_membarrier(), which
> executes a memory barrier on all threads running on the system. It is
> implemented by calling synchronize_sched(). It can be used to distribute
> the cost of user-space memory barriers asymmetrically by transforming
> pairs of memory barriers into pairs consisting of sys_membarrier() and a
> compiler barrier. For synchronization primitives that distinguish
> between read-side and write-side (e.g. userspace RCU [1], rwlocks), the
> read-side can be accelerated significantly by moving the bulk of the
> memory barrier overhead to the write-side.
>
> ...
>

It would be nice to hear about the real world value of this syscall to
our users. I'm seeing test results for a microbenchmark but so what.
What actual applications or application classes are calling for this and
what results can they expect to see?

>
> membarrier(2) man page:
> --------------- snip -------------------
> MEMBARRIER(2) Linux Programmer's Manual MEMBARRIER(2)
>
> NAME
> membarrier - issue memory barriers on a set of threads
>
> SYNOPSIS
> #include <linux/membarrier.h>
>
> int membarrier(int cmd, int flags);
>
> DESCRIPTION
> The cmd argument is one of the following:
>
> MEMBARRIER_CMD_QUERY
> Query the set of supported commands. It returns a bitmask of
> supported commands.
>
> MEMBARRIER_CMD_SHARED
> Execute a memory barrier on all threads running on the system.
> Upon return from system call, the caller thread is ensured that
> all running threads have passed through a state where all memory
> accesses to user-space addresses match program order between
> entry to and return from the system call (non-running threads
> are de facto in such a state). This covers threads from all pro___
> cesses running on the system. This command returns 0.
>
> The flags argument needs to be 0. For future extensions.
>
> All memory accesses performed in program order from each targeted
> thread is guaranteed to be ordered with respect to sys_membarrier(). If
> we use the semantic "barrier()" to represent a compiler barrier forcing
> memory accesses to be performed in program order across the barrier,
> and smp_mb() to represent explicit memory barriers forcing full memory
> ordering across the barrier, we have the following ordering table for
> each pair of barrier(), sys_membarrier() and smp_mb():
>
> The pair ordering is detailed as (O: ordered, X: not ordered):
>
> barrier() smp_mb() sys_membarrier()
> barrier() X X O
> smp_mb() X O O
> sys_membarrier() O O O
>
> RETURN VALUE
> On success, these system calls return zero. On error, -1 is returned,
> and errno is set appropriately. For a given command, with flags
> argument set to 0, this system call is guaranteed to always return the
> same value until reboot.

I suggest "with flags argument set to MEMBARRIER_CMD_QUERY" here.

>
> ERRORS
> ENOSYS System call is not implemented.
>
> EINVAL Invalid arguments.
>
> ...
>
> +SYSCALL_DEFINE2(membarrier, int, cmd, int, flags)
> +{
> + if (flags)
> + return -EINVAL;

I'm not a huge fan of this "add a flags arg to syscalls" rule. Is
there any realistic expectation that we'll ever *use* this thing? If
not, why add it?

You may as well put an unlikely() in there btw.

> + switch (cmd) {
> + case MEMBARRIER_CMD_QUERY:
> + return MEMBARRIER_CMD_BITMASK;
> + case MEMBARRIER_CMD_SHARED:
> + if (num_online_cpus() > 1)
> + synchronize_sched();
> + return 0;
> + default:
> + return -EINVAL;
> + }
> +}



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